In the field of axial fans, many different technical solutions are known for connecting a blade to the hub of the fan.
In order to briefly discuss the prior art, it is important to appreciate the static and dynamic forces acting on the blades of an industrial axial fan during its operation, and, in particular, with respect to the effect of the centrifugal forces acting on the fan.
Then, we will describe various solutions actually known in the field that have the purpose to decrease the steady and-or the unsteady loads on the element connecting the airfoil profile to the hub of the axial fan.
The forces acting on the blades of an axial fan during operation can be divided in steady forces A) and unsteady forces B).
A) The steady forces, as indicated in the attached FIG. 1a, are the following:
lift force (L);
drag force (D);
centrifugal force (C);
blade weight (W).
If the blade axis has an angle α with respect to the ideal rotating plane, the centrifugal force clearly will be split in two components, one radial and one perpendicular to the blade plane, having direction opposed to the direction of the lift, as shown in the FIG. 1b. 
Consequently, the force perpendicular to the blade plane will be reduced according to the following formula:T=L+W·cos ∝−C·sin ∝  (1)
B) The unsteady forces, are those generated by:
the interaction between the aerodynamic field created around the blade and the structure supporting the fan or the housing comprising it; they are proportional to the steady aerodynamic forces;
the operation at critical conditions such as blade resonance or structure resonance; their amplitude varies depending on the passive and active damping properties of the blade;
the interference with the environment like wind or other equipment;
the vortex wakes created by the blade profile, so that they are self-induced.
Their amplitude cyclically repeats, for that reason they are also commonly called alternated forces.
These forces originate fatigue phenomena, therefore are more dangerous for the blade life when compared to the steady forces.
The attachments actually known in the field and used on the market to decrease the steady and/or the unsteady loads generated by the forces acting on the blades during operation, are represented in the attached FIGS. 2a, 2b, 2c and 2d, and will be herewith briefly described.
A blade comprising a first kind of attachment comprises the rigid connection shown on FIG. 2a: a stiff attachment is used, having a stiffness in the radial direction higher than that of the profile.
The support of the attachment of the blade to the hub is designed so that the airfoil axis is inclined in the vertical plane and has a fixed angle α with respect to the ideal rotation plane. This arrangement as the vertical component of the centrifugal force is opposing the lift, is allowing to decrease the steady loads according the above mentioned formula (1), but has no effect on the unsteady loads.
Another kind of blade attachment known in the art comprises the hinged connection shown on FIG. 2b: a hinge with horizontal axis is acting as a connection between the airfoil and the hub. In this case the airfoil is free to rotate perpendicularly to the fan rotation plane, therefore when the fan is in operation it tends to keep a position where the traction force is balanced by the centrifugal force, minimizing the steady and unsteady loads.
A further blade attachment known in the art comprises a flexible connection constituted by one single element separated from the airfoil, as shown on FIG. 2c, connecting the airfoil to the hub, which has such a high flexibility that it can bend in the vertical plane without being overstressed, reducing both steady and unsteady loads.
Again, a further kind of attachment comprises a flexible connection constituted by two overlapping elements, separated from the airfoil, as shown on FIG. 2d, connecting the airfoil to the hub, which are interacting each other and will bend in a controlled way, in the vertical plane, without being overstressed. Steady and unsteady loads will be reduced.
It must be underlined that in the above described systems, which are today used in the industries, the blade tends to deform or is deforming under the alternated operating loads, modeling a shape (see displacement FIG. 3a) similar to that of a cantilever beam vibrating according to the first vibration mode (see FIG. 3b). This characteristic is entailing that the energy introduced in the blade and consequently transmitted to the hub and then to the structure is amplified. In fact the energy deforming the blade is calculated as follows:E=∫r1r2{right arrow over (f)}(r)·{right arrow over (ds)}  (2)
Wherein:
{right arrow over (f)}(r) is the traction force vector at a given radial section r;
{right arrow over (ds)} is the displacement vector of the system at a given radial section r;
r1 is the radial position of the connection between the blade and the hub;
r2 is the blade tip radial position.
In order to better explain the above equation (2), in FIG. 3b it has been schematized the first mode of vibration of a cantilever beam. As it is well-know, the scalar product of two vectors has a positive sign when both of them have the same direction, but it has a negative sign when they have opposite directions; the module of the product is proportional to the amplitude of the two vectors. For the first vibration mode, the displacement s(r) is a monotonically increasing function along the radial span; the applied alternated force f(r) is also a spanwise monotonically increasing function. Therefore, for a system vibrating as the first mode, the integral of equation (2) is the sum of only positive numbers, exponentially increasing in amplitude along the distance (r).
The blades known in the art, especially the ones of FIGS. 2a and 2c, and 2d, under alternated loads have a typical deformed shape of the blade similar to that of a cantilever beam vibrating according the first vibration mode, where the displacement ds of each blade single section increases along the radial span and the alternated force f(r) also increases accordingly. Additionally, both these parameters, the section displacements and the exciting forces, are changing their direction in phase.
Consequently it can be concluded that in such known systems the energy introduced into the blade is amplified and the loads acting on the attachment as well.